The alert for the whole country was raised a week ago following the Paris attacks to level three of the highest four, implying a "possible or probable" threat. Previously, only certain sites, such as the US embassy, were at level three.

A statement on the crisis centre's website called on local organisers to cancel major events and football matches over the weekend.

Metro services in Brussels were also cancelled over the weekend.

Al Jazeera's Paul Brennan in Brussels said despite the metro suspension, bus and tram services continued to run as normal.

"You would expect it to be slightly quieter on the weekend, and it is, but I have to say people are being rather stoic around here, and they don't seem to have a huge amount of fear.

"There's great concern in the wake of the Paris attacks; there's great concern it will happen elsewhere, especially because of the manhunt that's currently ongoing for one of the alleged Paris attackers, Salah Abdesalam."

"He [Abdesalam] is believed to be in the Brussels area ... and I think we can put two and two together that because he could be in the area, the government has decided to raise the threat level," Brennan said.

Raffaello Pantucci, director of international security studies at the Royal United Services Institute, told Al Jazeera that Belgian authorities likely based their decision on specific information.

"They're likely dealing with intelligence on some sort of offshoot group, and there's also a suspect [Abdelsalam] on the loose," Pantucci said.

"In some ways security measures being taken are an overreaction. Clearly we've seen across Europe a number of security scares this week… You do have these kinds of security responses after terrorist atrocities."